The coating of hollow fiber substrates with a polymer to make a composite membrane structure has generally been achieved by immersing the hollow fiber substrate in a pool of low viscosity and generally very dilute solution of a polymer in a volatile solvent. The hollow fiber substrate is then withdrawn and air-dried to form a film. Low viscosity coating solutions are typically used because high viscosity solutions tend to form non-uniform coatings on the substrate surface. When low viscosity solutions are used for coatings, the polymer concentration is not large enough to form a continuous film on a substrate with high porosity and large pore sizes due to shrinkage of the polymer coating. To achieve a good film, the substrate must be coated several times in succession or a vacuum must be applied on the bore side of the substrate so that by loss of solvent in the vicinity of the membrane wall, an increased solution concentration (and, hence viscosity) is obtained to form a desired film. The multiple coating approach yields films that are very thick and offers substantial resistance to mass transfer of the permeating component. In the vacuum method, thin coatings are achieved; however, this method must be practiced in a batch mode because discrete sections of fibers must be used to achieve vacuum through the bore. Both of the above methods fail to give uniform coatings when polymeric solutions having high viscosity and low solid contents are used.
U.S. Pat. No. 4,214,020 discloses a process for coating the exteriors of a plurality of hollow fibers which are suitable for fluid separations and which are assembled in the form of a bundle. The process involves immersing the bundle of hollow fibers in a coating liquid containing material suitable for forming the coating and a substantial amount of solvent. A pressure drop from the exteriors to the interiors of the hollow fibers is provided to result in the formation of deposits on the exteriors of the hollow fibers. The process enables essentially the entire exterior surface of the hollow fibers to be coated without undue sticking of the hollow fibers and avoids the formation of undesirably thick coatings on the hollow fibers in any portion of the bundle. While the disclosed process is capable of treating a bundle of fibers simultaneously, the process nonetheless is still a batch process, and is not operated as a continuous, on-line process.
U.S. Pat. No. 4,444,662 discloses a laminate formed by the solvent casting of a two-phase siloxane-polyarylene polyether block copolymer onto a suitable microporous substrate, such as a microporous propylene film, to produce a gas permeable and blood compatible membrane having sufficient mechanical strength for use in blood oxygenators and gas separation devices. The process for producing the laminate consists of a meniscus dip coating technique to apply a uniform coating of the polymer to only one side of the microporous substrate in order to maintain adequate coating thickness and to leave the other side uncoated for ease of heat sealing or potting of the membranes together into envelopes.